← all cardsTopic 11 · Wave Motion · Definitions
Intensity, I
● VERBATIM — examined word-for-wordThe rate of energy transfer (power) per unit area normal (perpendicular) to the direction of energy transfer of the wave. The perpendicular clause carries a mark.
I = P/AWhat this actually means
Intensity is power spread over area: how much energy per second passes through each square metre held face-on to the wave. Units W m⁻².
'Normal (perpendicular) to the direction of energy transfer' is a marked phrase. Tilt the area and less energy passes through it; the definition fixes the orientation so the number is well defined.
Same sunlight, bigger detector: more power collected, identical intensity. Intensity describes the location, not the instrument.
Prove it — watch it be true
- Set a detector distance and note the intensity readout
- Double the detector area: power collected doubles, intensity does not move
- Intensity belongs to the place; power belongs to the detector